In the operation of a typical chlor-alkali plant, the chlorine gas recovered from the electrolytic cells is dried to remove the water vapor carried out with the chlorine gas. The dried cell gas typically contains from about 1 to about 3 weight percent of noncondensables and most frequently from about 1.2 to about 1.5 weight percent of noncondensables. These noncondensables include hydrogen, oxygen, carbon dioxide, and nitrogen. Generally, the noncondensables include from about 0.2 to about 0.3 weight percent hydrogen, from about 50 to about 70 weight percent oxygen, from about 0 to about 20 weight percent carbon dioxide, and from about 20 to about 30 weight percent nitrogen. The chlorine gas generally received from the dryers is at a temperature of from about 10.degree. to 40.degree. Centigrade and at a pressure of from about 0.5 atmosphere to about 5.0 atmosphere.
Generally, in a high pressure system, chlorine is compressed in a multi-stage centrifugal compressor with interstage cooling, for example, with cooling tower water or any other suitable coolant, between every one or two or three or four or more stages. Thus, in a typical modern plant the dried chlorine-containing gas is delivered to the chlorine compressor at a temperature of about 29.degree. Centigrade and at a pressure of about one atmosphere. After the first two stages of compression, the temperature is about 115.degree. Centigrade and the pressure is about 1.5 atmospheres and the compressed gas is cooled. After the fourth stage of compression, the temperature is about 121.degree. Centigrade, the pressure is about 3.1 to 3.2 atmospheres, and the compressed chlorine gas is cooled, for example, by cooling tower water or other suitable coolants, to a temperature of about 35.degree. to 40.degree. Centigrade. It can then be subjected to further stages of compression, for example, to two additional stages, yielding a pressure of about 6.0 to about 6.1 atmospheres and a compressor outlet temperature of about 121.degree. Centigrade. An additional intercooler may be provided, lowering the temperature to about 35.degree. to about 40.degree. Centigrade, permitting further compression to a pressure of about 12.5 to 12.7 atmospheres and recovering therefrom a pressurized gas mixture at a temperature of about 115.degree. Centigrade. The compressed chlorine gas mixture obtained thereby may be readily liquefied, for example, by passing through a condenser to obtain a cooling of the chlorine to about 35.degree. Centigrade or lower thereby liquefying a significant fraction of the chlorine.
After liquefaction of the chlorine, a second gas mixture is present in the condenser and recovered therefrom. This second gas typically contains, depending on the pressure and temperature of condensation of the chlorine, from about 5 to about 20 percent noncondensables and most frequently from about 8 to about 15 percent noncondensables. Noncondensables are typically hydrogen, oxygen, carbon dioxide, nitrogen, and traces of other gases. According to the more common practice in the prior art, the chlorine is typically condensed out of the second gaseous mixture by cooling the second gaseous mixture with refrigerants such as fluorinated hydrocarbons. However, it is also known to condense the chlorine out of the second gas mixture by using the condensed chlorine to cool and liquefy the chlorine in the second gas mixture and then use the newly vaporized chlorine coolant as the feed to the first stage of the compressor. See, for example, United Kingdom patent 353,704 to Krebs & Company and United Kingdom patent 1,164,069 to Shipton. Similarly, it is also known to both utilize a fraction of the liquid chlorine as a coolant and to expand the second gas mixture slightly so as to affect some cooling thereof and some liquefaction thereof. See, for example, U.S. Pat. No. 3,230,724 to Havas.
However, in all of the processes of the prior art utilizing liquid chlorine as a coolant, only a single cooling stage is disclosed and the vaporized coolant is recycled to the first compressor in the compressor train.